The invention relates primarily to: (1) the preparation of materials that are useful precursors to ceramic materials; (2) catalytic activation of Si--H bonds; and (3) preparation of ceramic materials using the ceramic precursors of the invention in combination with metal and/or ceramic powders. The invention also concerns, in one embodiment, the use of sol-gel processing techniques.
The sol-gel process is an important route for advanced metal-oxide glasses and ceramics. The method is currently used or of potential for protective, optical and electronic coatings, optical fiber preforms, nonlinear optical devices, dielectrics or superconductors, display materials, and structures. The sol-gel technique provides a relatively low temperature, controlled method of producing a large variety of shapes such as monodispersed particles, uniform coatings, fibers, dense or porous articles, and mixed metal oxides having controlled stoichiometry and purity at the molecular level.
The sol-gel process has been based mostly on the same group of starting materials, the metal alkoxides, carboxylates and diketonates. These precursors are hydrolyzed, then condensed in the presence of an alcohol/water solution to form a gel which is dried and fired to give the final product. Chemical control of product formation is manipulated by temperature, type of catalyst and pH as well as by the type and ratio of reactants in solution. See, e.g., C. J. Brinker et al., in "Ultrastructure Processing of Ceramics, Glasses and Composites I" (1984), at pp. 43 et seq.
Thus, the reaction procedure controls to a large extent the morphology of the final gel, and, therefore, the final ceramic microstructure as well. Low water content and/or acidic conditions will give spinnable gels because the precursor polymer will, as noted above, be substantially linear. Higher water content will give slightly crosslinked, coatable gels, while a very high water content and/or basic conditions will give highly crosslinked gel products that are useful in casting processes and for powder formation. See B. J. J. Zelinski et al., J. Phys. Chem. Solids 45:1069 (1984), and L. C. Klein et al., Ann. Rev. Mat. Sci. 15:227 (1985).
It has recently been suggested that alkoxide-siloxane oligomers may serve as molecular building blocks for unique ceramic silica structures (V. W. Day et al., J. Am. Chem. Soc. 107:8264 (1985)). A rigid cubic alkoxysesquisiloxane, [Si.sub.8 O.sub.12 ](OCH.sub.3).sub.8, offers the possibility of generating porous materials, yet rigid due to the molecular block structure.
As noted above, the invention also relates to preparation of preceramic polymers, i.e., polymers which may be converted upon pyrolysis to ceramic products. The present invention provides preceramic siloxane polymers which are useful for preparing a wide variety of silicious ceramic materials and articles, e.g., articles such as fibers, films, shaped products, and the like, comprising materials such as silica, silicon oxynitride, silicon carbide, or metal silicate.
The preceramic polymers, or "ceramic precursors," of the invention are prepared by catalytic activation of Si--H bonds. To date, catalytic activation of Si--H bonds has mainly been used for hydrosilylation reactions of unsaturated compounds, as illustrated by reactions (1) and (2): EQU R.sub.3 Si--H+M.fwdarw.R.sub.3 Si--M--H (1) EQU R.sub.3 Si--M--H+R.sub.2 C=X.fwdarw.R.sub.2 CH--XSiR.sub.3 +M(2)
(X=O, CR.sub.2)
Over the past 25 years, numerous homogeneous and heterogeneous catalysts have been found which promote these reactions. See, e.g., J. L. Speier et al., J. Am. Chem. Soc. 79:974 (1957). Typical applications of these reactions have been in organic synthesis or in the crosslinking of silicon rubbers (J. P. Collman et al., in "Principles and Applications of Organotransition Metal Chemistry," pp. 384-392, University Science Books, 1980). Only recently have such reactions been found useful in another application, crosslinking of pre ceramic polymers, as described in co-pending, commonly assigned application Ser. No. 012,874, the disclosure of which is hereby incorporated by reference in its entirety.
Related reactions involving substitution at an Si--H bond have been used to form compounds containing Si--Y groups wherein Y is, for example, halogen, alkoxy, or substituted or unsubstituted amino: ##STR1## L. H. Sommer et al., J. Org. Chem. 32:4270 (1967). Only mono- and di-substituted aminosilanes, halosilanes and alkoxysilanes have been synthesized by this method. Surprisingly, there have been virtually no attempts to enlarge the potential capability of reaction (3). For example, the inventors herein are unaware of any work involving reaction of compounds containing multiple Si--H bonds with water to form oligomeric or polymeric siloxane products.
Investigators at SRI, the assignee of the present application, have discovered that catalytic activation of Si--H bonds is extremely useful in the synthesis of polysilazane ceramic precursors, according to reaction (4): ##STR2## To date, however, efforts have not been focused on enlarging the scope of the analogous reaction in the presence of water, i.e., instead of using ammonia or monoalkylamines. Preliminary research indicates that similar reactions (as illustrated by reactions (5) and (6)) will occur in the presence of water, to produce monomeric, oligomeric or polymeric siloxanes, at room temperature, or lower: ##STR3##
The present invention is directed to a new approach to polymer processing, and involves combining the fields of research summarized hereinabove: (a) preparation of preceramic materials, particularly preceramic polymers, useful in making ceramic materials; and (b) reaction of hydridosiloxane compounds by catalytic activation of the Si--H bonds contained therein. In a preferred embodiment, the invention also involves the use of (c) sol-gel processing techniques. Gels or ceramic precursors produced using the present method are highly "processable" and, upon pyrolysis, give the desired ceramic material in relatively high yield.
The invention is also directed to a novel method for preparing ceramic materials which involves admixture of the gels or ceramic precursors produced using the present method with a metal powder, a ceramic powder, or mixtures thereof, prior to conducting pyrolysis. This method can be used, for example, in the preparation of mullite, 3Al.sub.2 O.sub.3 .cndot.2SiO.sub.2. Although mullite is a naturally occurring mineral, it is generally formed either as a product of heated aluminosilicate or synthetically from mixtures of aluminum and silicon sources. Such techniques are, however, time consuming and require many preparation steps. The method of the invention, by contrast, provides a simple, straightforward way of making materials such as mullite or mullite-containing composites, and further provides a method for making such materials in the form of coatings, composite matrices, structural monolithic ceramics, and the like.
In addition to the references mentioned above, the following relate to one or more aspects of the present invention, and reference may be had thereto for background information not explicitly included herein.
U.S. Pat. No. 5,225,283 to Leung et al. describes a process in which a cyclosiloxane monomer containing a filler is applied as a coating to a substrate and then pyrolyzed. The filler may be silicon carbide, silicon nitride, or the like, in the form of a powder, fibers or whiskers.
U.S. Pat. Nos. 5,231,059 and 5,266,533 to Leung et al. relate to a process for preparing black glass composites by reacting cyclosiloxane monomers in the presence of reinforcing fibers, such as of silicon carbide, silica glass, silicon nitride, or the like, and then pyrolyzing the mixture.
U.S. Pat. No. 5,306,554 to Harrison et al. relates to the preparation of fiber-reinforced ceramic matrix composites containing ceramic particles such as oxides of aluminum, silicon, calcium and zirconium.
PCT Publication No. WO94/05675 describes preparation of a carbon-containing black glass synthesized by heating a spirosiloxane polymer in a non-oxidizing atmosphere. The process is also stated to be useful in the preparation of matrix composites by conducting pyrolysis in the presence of a filler such as of silicon carbide, silicon nitride, silica, etc.
Claussen et al., J. Eur. Ceram. Soc. 5:29-35 (1989) and 9:97-109 (1994), describe a method for making aluminum oxide-based composites by heat treating an aluminum/aluminum oxide powder to yield aluminum oxide crystallites. The aluminum oxide crystallites in turn sinter and bond the originally present ceramic particles.
Erny et al., J. Am. Ceram. Soc. 76(1):207-213 (1993), relates to preparation of ceramic composite materials using polysiloxane precursors and titanium powder.